3-4, 4-3 OVER

Question
What exactly does it mean? also how do they understand the play when the QB say gibber gabber stuff?

Answer
The 3-4 and the 4-3 are the base alignments almost all NFL defenses play, and the names refer to the number of defensivelineman and linebackers on the field (otherwise known as "the front seven").

A 3-4 is 3 lineman and four linebackers.  This alignment requires the linemen to engage blockers to free up the linebackers to make plays - the NE Patriots are an example of a base 3-4 team.

The 4-3 is a more common alignment, with four defensive and three linebackers.  In the 4-3, the lineman are required to penetrate and shoot into the offensive backfield to make plays.  The Over/Under designation in a 4-3 (4-3 Over) is a shift to the strongside of the offense - your defense loads up on their stongside.  The Under designation means there is a shift to overload the weakside.

I have no idea what you mean by "how do they understand the play when the QB say gibber gabber stuff?"